A longer bulb may contact the headlight lens before it's fully seated.

Also, the headlight bucket's concave mirror optics are designed such that the halogen filament must be correctly positioned at the proper distance for maximum light output.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car